Ferrofluids (sometimes referred as magnetic liquids) are colloidal suspensions of magnetic nanoparticles, represent a special class of magnetic fluids and are manufactured fluids consisting of dispersions of magnetized nanoparticles in a variety of non-magnetic liquid carriers. They were originally invented independently around the same time in the early 1960s at NASA Lewis Laboratories, and aslo by Dr. R. E. Rosensweig et al. at AVCO Space Systems. Typically, particles within such colloidal suspension are about 10 nanometers (nm) in diameter and suspended in either water or oil.
They are stabilized against agglomeration by the addition of a surfactant monolayer onto the particles. In the absence of an applied magnetic field, the magnetic nanoparticles are randomly oriented, the fluid has zero net magnetization, and the presence of the nanoparticles provides a typically small alteration to the fluid’s properties such as viscosity and density.
When a sufficiently strong magnetic field is applied, the ferrofluid flows toward regions of the magnetic field, properties of the fluid such as the viscosity are altered, and the hydrodynamics of the system can be significantly changed.
Since the first successful production of stable ferrofluids in the early 1960s (Papell 1964) the field of ferrofluid research developed quickly in different branches:
- Physics: connected to the fundamental description and characterization.
- Chemistry: as basis for ferrofluid preparation.
- Engineering: to prepare and provide application.
The field of ferrofluid research is relatively young compared to the investigation that have been done in fluid dynamics in general. The famous book “Ferrohydrodynamics” by Rosensweig 1985 is one of the standard textbooks in this field which must be mentioned here. It covers various areas in this research field, synthesis and properties of magnetic fluids, foundation of ferrohydrodynamics theory, hydrodynamics in ferrofluids, as well as problems and applications.
However, the term ferrohydrodynamics was established first by Neuringer and Rosensweig 1964. This includes the continuum description of the flow behavior of magnetic fluids in the presence of magnetic fields. Later Shliomis 1972 developed a theory including the experimental findings of magnetoviscous effects by Rosensweig et al. 1969 and McTague 1969. Further to mention is the book “Magnetic Fluids” by Blumes et al. 1997 which focuses on the rheology of ferrofluids in more detail, also including theoretical discussion of the magnetoviscous effect, rotational viscosity variation of shear rate, and many more.
In this context also to mention is the earlier work by Blumes et al. 1986, which despite being mainly devoted to conducting fluids and the action of Lorenz forces, also elucidates the effects of heat and mass transfer in ferrofluids. The application of ferrofluids and magnetic fluids in general is summarized in the books by Berkovsky and Bashtovoy 1993 & 1996.
They provide a wide overview of various possible uses of ferrofluids in different fields/areas, reaching from separation over mechanical positioning towards medical applications. Nowadays, ferrofluids are utilized in a wide variety of applications, ranging from their use in computer hard drives and as liquid seals in rotating systems to their use in laboratory experiments to study geophysical flows and the development of microfluidic devices.
Any kind of fluid which can by externally controlled, e.g. by a magnetic field represents a challenging subject either for scientists interested in basic fluid mechanics/dynamics as well as application engineers. Consider basic research: the ability of introducing an artificial external but controllable force into the basic equations reaches out into a fascinating field of potential new phenomena. The fact that magnetic fields can be varied quite well and accurately, both in direction/orientation and field strength, makes them highly interesting for adding such external forces. Unfortunately (most normal) natural liquids do not offer these features. However, artificial generated suspensions of magnetic nanoparticles in appropriate carrier liquids, i.e. ferrofluids, do so. Although various different effects have been discussed to date, by far the most famous field-induced property of magnetic fluids is the change of their viscosity (McTague 1969).
In general, a magnetic field can be used effectively as a control or bifurcation parameter of the system, whose change can lead to characteristically distinct types of hydrodynamical behavior. In this regard, turbulence and transition to turbulence in Magnetohydrodynamic (MHD flows) play an important role in many astrophysical and geophysical problems, e.g. the generation of magnetic fields in heavenly bodies, in planets and (sometimes) in large-scale industrial facilities. For instance, Gellert et al. 2011 studied current-driven instabilities of helical fields.